Phase 5 had never been declared since the warning system was introduced in 2005 in response to the avian influenza crisis. Phase 6 means a pandemic is under way.

The health organization said its decision was based on the continuing spread of swine flu in the United States and Mexico, particularly the increasing numbers of unexplained cases among people not exposed to travelers or to institutions like schools or hospitals where many people have close contact with one another and high rates of transmission might be expected.

“All countries should immediately activate their pandemic preparedness plans,” Dr. Margaret Chan, director general of the organization, said at a news conference in Geneva. “Countries should remain on high alert for unusual outbreaks of influenza-like illness and severe pneumonia.”

Dr. Chan emphasized the need for calm, but at times spoke as if a pandemic had already begun, saying, for instance, “W.H.O. will be tracking the pandemic.” She also emphasized that developing countries tended to have more severe flu epidemics than rich ones, and said her organization and others would need to make special efforts to help poorer nations.

She called for global solidarity, saying, “After all, it really is all of humanity that is under threat during a pandemic.”

President Obama, terming the outbreak “cause for deep concern but not panic,” took the unusual step of using a prime-time televised news conference, convened to mark his 100th day in office, to deliver a public health message to the American people.

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Commuters wore protective masks at a subway station in Mexico City on Wednesday.Credit
Daniel Aguilar/Reuters

“Wash your hands when you shake hands, cover your mouth when you cough,” he said from the East Room of the White House. “It sounds trivial, but it makes a huge difference. If you are sick, stay home. If your child is sick, take them out of school. If you are feeling certain flu symptoms, don’t get on an airplane.”

With public health officials recommending that schools close if there are more confirmed or suspected cases, Mr. Obama urged parents and businesses to “think about contingency plans” in case of such closings.

He said he was calling on Congress to authorize an immediate $1.5 billion to “support our ability to monitor and track this virus” and to build the supply of antiviral drugs.

“The more recent illnesses and the reported death suggest that a pattern of more severe illness associated with this virus may be emerging in the U.S.,” the C.D.C. said on its Web site. More hospitalizations and deaths are expected, the site said, because the virus is new and most people have no immunity to it.

Dr. Chan said that government preparedness plans could include steps like ensuring that laboratories can test for the disease and that health systems can identify and treat cases, track an outbreak and prevent the virus from spreading in hospitals and clinics. She said governments should also decide on measures like closing schools and discouraging or banning public gatherings.

Mexico, for instance, has prohibited people from eating in restaurants and ordered most stores and other businesses to close for several days starting Friday, a move apparently intended to keep people at home during what is traditionally a long holiday weekend.

At a news conference in Mexico City on Wednesday night, Mexico’s secretary of health, José Ángel Córdova, also announced that the national government would close all but essential offices during the same time period. The government did not suspend mass transportation or close airports and asked that supermarkets and pharmacies remain open.

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In the United States, Dr. Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, said the outbreak had caused such concern because officials had never seen this particular strain of the flu passing among humans.

“There is no background immunity in the population, and it is spreading from human to human — all of which has the potential for a pandemic,” Dr. Fauci said.

Dr. Richard Besser, acting director of the Centers for Disease Control and Prevention, said that officials had no way of predicting whether the outbreak would become more serious.

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Margaret Chan, left, director general of the World Health Organization, and Anarfi Asamoa-Baah, the deputy director, announced a raising of the swine flu alert level on Wednesday in Geneva. Credit
Martial Trezzini/European Pressphoto Agency

“You don’t know if this is a virus that will fizzle in a couple of weeks or one that will become more or less virulent or severe in the diseases it causes,” Dr. Besser said.

He said officials must follow government plans for a pandemic because of that unpredictability.

“If we could see into the future, it would be wonderful so that we could tailor all our responses specifically to what is occurring,” Dr. Besser said.

The disease centers’ count of 91 confirmed cases in the United States did not include some later reports by states that confirmed cases after the C.D.C. tally was posted. In addition, there were suspected cases in Louisiana and Delaware. Kits being provided to the states and other countries will allow them to test for the virus on their own and obtain results within a few hours.

New York City added 5 new confirmed cases, bringing its total to 49. All have links to Mexico or St. Francis Preparatory School in Queens, where the virus first surfaced in New York, health officials said. The city identified five more probable cases.

The total in Canada rose to 19, from 16. In Mexico, the number of confirmed cases of the flu rose to 99 from 49, and the number of deaths from confirmed cases of the flu was increased to 8. The number of suspected cases is much higher.

Kathleen Sebelius focused on the outbreak on Wednesday during her first news conference as the Obama administration’s secretary of health and human services. “We’re determined to fight this outbreak and do everything we can to protect the health of every American,” Ms. Sebelius said.

She noted that the Centers for Disease Control and Prevention had recommended that schools close only if a student is found to be infected. More aggressive steps are under discussion, Ms. Sebelius said, but officials realize that closings can cause problems for families.

“What happens to parents? Where do children go?” she asked.

Dr. Besser, who joined the news conference via a video feed, said the most recent cases included patients of a broad range of ages, with two-thirds of all cases occurring in people under 18.

“There have been five hospitalizations so far, including the child who died. But we have a number of suspect cases that have been hospitalized and we expect that number to go up,” he said. Dr. Besser said that a quarter of the nation’s stockpile of 50 million treatments of antiviral medicines would be distributed to states by Sunday.

The United States has no plans to close international borders because, Dr. Besser said, such closings are not effective in slowing pandemics. When Hong Kong was hit with severe acute respiratory syndrome, or SARS, “increased border screening on entry and exit was not an effective way of identifying cases or preventing transmission,” he said.

Some elected officials have begun to question the decision to leave the borders open. Homeland Security Secretary Janet Napolitano was grilled by senators on Wednesday who asked whether her agency was doing enough to stop the virus from spreading from Mexico. The senators, including John McCain of Arizona and Joseph I. Lieberman of Connecticut, asked several times why the administration had decided against closing the border and banning travel to Mexico.